1. Field of the Invention
This invention relates to a process for the fabrication of micro hinges by a surface micro-machining method. More particularly, it relates to a process that enables the fabrication of self-aligned micro hinges on silicon on insulator (SOI) wafers. The unique silicon structure can be used in a micro-electro-mechanical system (hereinafter xe2x80x9cMEMSxe2x80x9d) that requires free-rotating hinged micro elements.
2. Description of the Prior Developments
Microdevices based on micron and millimeter scale microelectromechanical systems (MEMS) technology are widely used in microsensors, microfluidic controls systems, and micromachines. Currently, MEMS sensors are used in a very wide variety of fields such as automobiles, medical instrumentation and process control applications. In such use accurate determinations of pressure, temperature, acceleration, gas concentration, and many other physical or chemical states can be provided by tightly packed tiny sensors. In part because of the ease of associating and integrating electronic control circuitry (using VLSI techniques), microdevices are commonly constructed from semiconductor material substrates such as crystalline silicon, commercially available in the form of semiconductor wafers as used for the production of integrated circuits. Unfortunately, such wafer type substrates are limited in size in that they generally have a circular diameter of only about ten to twenty centimeters and less than one millimeter thick. Since many potential applications of MEMS require arrays of microdevices distributed on a centimeter scale in all three dimensions, significant costs are associated with the construction, distribution, attachment, and interconnection of large microdevice arrays based on silicon substrates. A low cost and reliable construction method that enables one to effectively and efficiently manufacture three dimensional MEMS is at the core of state-of-the-art research dealing with this type of technology.
One major branch of three dimensional (3D) MEMS evolves around the concept of micro hinges fabricated by a surface micro-machining method. The traditional microfabrication process was somewhat limited because it could only build strictly two-dimensionally (2D) extruded devices. Micro hinges revolutionized the microelectromechanical system (MEMS) world by permitting the fold-up of what was originally flat, two-dimensional parts. Parts are still built in-plane, but are hinged or chained together as built. At the end of the process, they are folded out-of-plane and latched together. Many complicated structures were built using this method, and the applications include micro optics (e.g. optical switches), sensors, and actuators. Examples of some of the first work that has been done in this area of technology are described in Sensors Actuator A, vol. 33, pp. 249-256, 1992 by K. S. J. Pister, et al. entitled xe2x80x9cMicrofabricated hingesxe2x80x9d, and a recent summary is in the Proc. of the IEEE, vol. 86, pp. 1552-1574, 1998 by J. Bustillo, R. T. Howe and R. S. Muller entitled xe2x80x9cSurface Micromachining for Microelectromechanical Systems.xe2x80x9d
However, the negative element about the technology described in these articles is that the original process techniques for fabricating micro-hinges or hinge joints by a surface micro-machining method, and the original hinge designs have many disadvantages. One particular disadvantage is the complexity of the process used for manufacturing these hinges. Known process techniques require deposition of at least five layers of film (e.g., three layers of polysilicon and two layers of silicon dioxide) along with the associated lithography techniques to complete the process. In particular, the fabrication process becomes extremely complicated if one attempts to directly integrate this process with a silicon on insulator (SOI) wafer.
There is illustrated in FIG. 1A, FIG. 1B and FIG. 1C a basic known technique for surface micromachining on a silicon on insulator (SOI) wafer which falls into the area of technology designated as the prior art. FIG. 1A illustrates how one starts with a layer of silicon 10 on an insulator 15 and a silicon substrate 11. FIG. 1B illustrates that a layer of material, i.e. silicon dioxide 12 is then deposited on the silicon. FIG. 1C then shows that a layer of a photoresist 14 material is then deposited on the silicon dioxide. The structure is then irradiated under a binary photomask, the photoresist is developed and excess photoresist material removed. Thereafter, one etches to remove exposed material 12, and/or layer 10 as well. Right after any remaining photoresist is stripped. All of these process steps are repeated for each of the structural and sacrificial layers. To build a floating hinge, three layers of polysilicon and two layers of silicon dioxide are needed.
The complexity grows exponentially as more layers are added if a proper connection between the layers are required. For example, only one step of photolithgraphy and etch is needed for the first layer. But for the third layer to connect to both the first and second layers, another etch is needed to open holes at the second layer. The associated cost of manufacturing goes up with the complexity of the process. Furthermore, the thickness and the exact number of layers of the various materials can create a large topography and stress problem. Large topography prevents proper lithography that defines the feature size. Stress can cause cracking problems.
A primary objective of the features of the present invention is to provide a fabrication process which avoids the above described disadvantages.
It is also a primary objective of the present invention to provide a simple approach to permit the construction of micro-hinges on silicon on insulator wafers then has previously been known before. It is an additional objective of the present invention to describe a surface micromachining process for the fabrication of three-dimensional micro-hinges with tighter hinge tolerance than exhibited by known structures because the hinge pivotable portion of the micro-hinge structure is self aligned to the hinge staple portion.
The above described objectives, and others, are obtained in accordance with the preferred features of the present invention by employing a surface micromachining process for the fabrication of three-dimensional micro-hinges directly on silicon on insulator wafers comprising the steps of: (a) defining openings around the surface of a desired hinge pin in a single layer of a silicon single crystal; (b) subjecting the openings to an etching process for removal of oxide material that is located in contiguous relation to the openings under the area designated for a hinge; (c) growing thermal oxide to define a gap between a hinge pin and a subsequently deposited polysilicon cap; (d) depositing polysilicon and etching to define a hinge cap/staple; and (e) wet etching to remove grown oxide and allow a pin to rotate inside the cap/staple. A connected mirror structure can be lifted out of the silicon wafer.